Field of the Invention
The invention relates to a method for injection duration correction in an internal combustion engine having a secondary-air system blowing air into an exhaust track upstream of a catalyst. The injection duration is controlled or corrected in dependence on a main-air mass flow, the secondary air-mass flow and a pilot control value for an air/fuel ratio.
The pollutant emission of an internal combustion engine can be reduced effectively by catalytic after treatment with the aid of a three-way catalyst in conjunction with a lambda control device. An important precondition for this, however, is that, as well as the lambda probe of the control device, the catalyst, too, has reached its operating temperature. Below this temperature, approximately 300.degree. C. in typical motor vehicle catalysts, the catalyst is ineffective to only slightly effective and the reaction takes place only at insufficiently low conversion rates (&lt;10%). Various warm-up strategies are known in order to ensure that the light-off temperature is reached quickly and consequently to reduce pollutant emission during the cold-starting phase of the internal combustion engine when approximately 70 to 90% of all the HC and CO pollutants are emitted within the first 10-15 seconds.
Rapid heating of the catalyst may be carried out not only by retarding the ignition angle, raising the idling speed and making the mixture leaner, but also by enriching the mixture and at the same time blowing secondary air into the exhaust tract of the internal combustion engine. In this case, during warm-up, secondary air is blown downstream of the outlet valves of the internal combustion engine via a secondary-air pump. Since the catalyst is operated with an oxygen excess due to the secondary air blown into the exhaust gas, the catalyst has an oxidizing action, that is to say the chemical reaction is exothermic, with the result that the intrinsic heating up of the catalyst is accelerated (see in this respect, for example, Kraftfahrtechnisches Taschenbuch/Bosch [Automobile Manual/Bosch], 22nd edition, 1995, pages 489 and 490).
However, the feed rate of the secondary air supplied cannot be increased as desired, since, beyond a specific mass of blown-in secondary air, cooling is brought about, which counteracts the heating and thereby delays the heating. It is therefore necessary to vary the secondary-air mass and the injection, that is to say the air/fuel ratio .lambda. of the internal combustion engine, optimally as a function of the secondary air mass.
Published, European Patent Application EP 0 469 170 A1 discloses a method for heating an internal combustion engine during warm-up, in which the quantity of the secondary air blown in is adapted as quickly as possible according to the operating state, so that a stoichiometric ratio is obtained. For this purpose, the feed rate of the secondary-air pump is controlled according to the enrichment factor and the intake air mass of the internal combustion engine or the feed rate of the secondary-air pump is set according to the load of the internal combustion engine in association with a temperature correction, regulation being superposed on a pilot control of the feed rate of the secondary-air pump. The regulation serves for correcting the feed rate pilot control that may possibly be maladapted over the course of time. The output signal from a lambda probe disposed in the exhaust tract serves as an actual-value variable for the regulation.